Word learning is a gradual process determined by the competencies of the word learner and cues provided by the word learning environment. Our long-term goal is to elucidate individual differences in both extant word knowledge and word learning among school-aged children. The proposed design takes advantage of naturally-occurring variation between populations. Our method involves comparisons within a group of children bringing intact social and linguistic processing to bear on word learning; between younger and older children with differing levels of maturity in social and linguistic processing; and between normal children and those with poor linguistic processing (children with specific language impairment) or poor social reasoning (children with autism). These comparisons allow tests of two main hypotheses: 1) the gradual or "slow-mapping" phase of word learning depends upon abilities to process a coalition of linguistic and social cues and 2) at any given point in time, the robustness of semantic knowledge established in the lexicon will affect the success of word usage. The specific aims are to determine: 1) how extant word knowledge varies with development, clinical status, and word class; 2) how receptive and expressive word usage vary with development, clinical status, and word class; 3) how the process and outcomes of slow mapping vary with development, clinical status, and word class; and 4) whether there is a causal relation between robustness of lexical semantic knowledge and speed or accuracy of word usage. This project will lead to discoveries about word learning during a rarely studied stage, slow mapping. It will also reveal sources of lexical deficits associated with SLI and autism, impairments affecting 7% and .3% of the U.S. population, respectively.